Making Polytarp Sails - With Duct Tape
The idea behind making polytarp sail with duct tape, is you are trying to quickly make a sail that works. There is no way that a tarp sail will last as long as a dacron one, BUT, if you take care of them, they will last for an adequate time and be fun to use, especially when you take into consideration how cheap and quickly they can be made.
Try to keep the duct tape dry, and store your sail in a place that the sun & weather won't beat down on it. When the sail breaks, repair with more duct tape or other materials. When breaks or worn beyond repair, make another tarp sail - will probably go faster this time since you already know what you are doing.
Make your spars first. This sail is a high peaked balanced lug sail. You can get a rough idea of how big your sail will be by tying line to your spars and stretching them out on the tarp. This will let you cut away the excess tarp, so you only have to work with a piece that is slightly larger than your final sail.
Use duct tape to stretch and hold the tarp to the ground.
Draw the perimeter line of your sail on the tarp. This sail's draft is being shaped with the curved luff and foot method.
Well, since it is a 4 sided sail, it has a curved head and foot.
You can either draw a nice curve to it, or you can use slightly less effecient straight lines like shown.
The leech (aft edge of the sail) is made by laying a single piece of duct tape along the inside of the line. This will be the aft edge of the sail. We just want one smooth strip of tape back there, so the air can flow and escape from the sail in a smooth motion.
The head, luff, and foot all are going to have a "bolt rope" sealed in their edges. The purpose of the rope is to absorb the tension and stress placed on those edges.
We start by laying a strip of double sticky carpet tape centered on the perimeter lines.
At this point we have tape all along the entire perimeter of our sail.
We can now cut away the excess material.
Time to install the bolt rope. Most polytarps come with a small line along the entire perimeter. It is made from a low stretch synthetic materials and is excellent for bolt rope. Cut the line out of the scraps that you have already removed, and make a loop in one end. Start at the peak, and lay along the middle of the carpet tape.
Fold the tape over as you go sealing the rope along the edge of the sail. Do this on the entire head, luff, and foot of the sail, ending in a small loop at the clew.
Lay down duct tape over the remaining seam.
Our sail is almost complete.
Instead of using grommets, you can use a soldering iron or other pointy heated object (like ice pick or screw driver heated with propane torch) to poke holes along the edges.
The spacing is up to you, I prefer to put them 10" apart. Some people lace as close as 6", others as wide as 12".
I use zip ties to lace my sails to my spars, but others use line. If using line, there are many different lacing patterns that can be done.
Presto, with about an hour of of work, you have a completed sail.